There are three types of passive display employing liquid filled cells, namely, liquid crystal displays, electrochromic displays and electrophoretic displays. By "passive display" is meant a display which is not light generating but is visible only when illuminated by an external source. All of these employ display electrodes, energisation of which causes an optically perceptible effect on or around the electrodes. Although many of these displays are viewed directly in natural light, there is often a need to provide additional illumination particularly if an image of the display is to be projected through a lens arrangement.
Some displays may be illuminated and viewed by transmitted light. An example of such an arrangement for a liquid crystal cell is shown in U.K. Pat. No. 1442249 (K.K. Daini Seikosha). However, the use of light transmission imposes considerable limitations on the structure and materials of the cell itself in that much of the cell must be transparent or at least translucent.
Accordingly, many displays are illuminated and viewed in reflected light. One arrangement for illuminating and projecting a liquid crystal display in reflection is shown in U.S. Defensive Publication T938006 (Stephany). Light from a collimated source is totally internally reflected by the oblique face of a right-angled prism onto a liquid crystal cell. Light reflected from the cell passes through the oblique surface and is focussed by a lens onto an imaging screen.
A report No. AD-A048198/6SL (August 1977) for the U.S. Government entitled "Liquid crystal Airborne Display" by R. N. Winner, M. N. Ernstoff and W. R. Byles, of Hughes Aircraft Corporation, shows the illumination of a liquid crystal display by means of a wedge prism of small apex angle placed on its side above the display. A source of light is provided near the base of the prism and it is shown that, after multiple internal reflections, bands of light emerge from the side faces of the wedge prism. The downwardly directed bands illuminate the cell so that it can be viewed in reflected light.
An article entitled "Improving the Efficiency and Image-Uniformity of a Simple Reflective Light Valve Projector" by A. G. Dewey (IBM Technical Disclosure Bulletin, Volume 22, Number 11, April 1980, page 5062) shows, inter alia, the use of a beam splitter to direct light from a condenser lens onto a "light-valve". Reflected light from the light valve then passes through the beam splitter to a projection lens.
The illumination systems proposed in the abovementioned prior art have the disadvantage, from the point of view of projection, of taking up space over the cell top which restricts the entrance pupil of the projection lens. A further specific disadvantage of the beam splitter arrangement is that the incident light intensity is reduced by a factor of approximately four by the beam splitter itself before it reaches the projection lens. The wedge and prism alternatives have the specific disadvantages of introducing an offset of the optic axis of the display and also producing chromatic aberration.